Quality of an optical signal transmitted through an optical transmission channel such as an optical fiber depends upon chromatic dispersion. Thus, in order to reduce waveform degradation of an optical signal caused by chromatic dispersion in long-distance optical transmission, one or a plurality of dispersion compensation modules is provided on an optical transmission channel. In this case, a compensation amount of each of the dispersion compensation modules is adjusted so that, e.g., residual chromatic dispersion of a path that optical signals of respective wavelengths are transmitted (called the “wavelength path” hereafter) through stays within a dispersion tolerance on a receiving end.
A method for deciding a compensation amount in accordance with a dispersion compensation map is disclosed as one of methods for designing chromatic dispersion compensation in an optical network. In this case, the dispersion compensation map is often provided for intervals between, voluntarily chosen, two points (or between, voluntarily chosen, two optical nodes) as usual. The compensation amounts of the dispersion compensation modules are set in the optical network in accordance with the dispersion compensation map, so that the waveform degradation caused by the chromatic dispersion of the wavelength path that the optical signals are transmitted through between the above voluntarily chosen two points can be reduced.
According to the method for deciding a compensation amount in accordance with the dispersion compensation map, however, an error between a design value of the chromatic dispersion of an optical transmission channel and a practical value of the chromatic dispersion or an error caused by discrete compensation amounts of the dispersion compensation modules exists, and the error accumulates depending upon the wavelengths. Further, even if a compensation amount of a dispersion compensation module is set between voluntarily chosen two points in accordance with the dispersion compensation map in an optical network where an optical add drop multiplexer (OADM) or a wavelength cross connect node is employed in recent years, a compensation amount which is not the optimum may possibly be set to a wavelength path set between other voluntarily chosen two points located between the above two points. Thus, it is difficult to implement chromatic dispersion compensation suited to a desired dispersion compensation map for every wavelength used on the optical network.
In order to solve the above problem, e.g., extract a plurality of wavelength paths from an optical network having complicated topology, a design method is disclosed in which all residual chromatic dispersion values are set within an allowable residual chromatic dispersion range set to all the wavelength paths at respective end points of the wavelength paths, and a dispersion compensation amount is provided for a dispersion compensation module for each of the wavelength paths. According to the design method, an optimum combination of candidate values of the compensation amount to minimize a summation of errors between the residual chromatic dispersion values and residual chromatic dispersion target values of the respective wavelength paths is decided so as to set the compensation amount of each of the dispersion compensation modules.
The compensation amount of chromatic dispersion means, in the specification, that a chromatic dispersion amount is compensated so that an optical signal having chromatic dispersion can be received adequately or somehow suitably enough not to cause a trouble in ordinary communication.
Japanese Laid-open patent Publication No. 2009-212565 is a related art.
The design method described above assumes an environment in which a plurality of kinds of candidate values of the compensation amount is set (or prepared, in some cases) in advance. It is necessary, in the above environment, to decide a presumably optimum one of lots of combinations formed by the plural kinds of candidate values. Thus, according to the design method described above, the presumably optimum one of the combinations is decided by the use of mixed integer programming. As the number of the candidate values of the compensation amount increases, however, the number of the combinations of the candidate values becomes huge. Thus, according to the above design method using the mixed integer programming, an amount of operations to decide the presumably optimum one of an enormous number of the combinations may swell enormously. Assume, e.g., an optical network in which 26 optical nodes are connected to one another via 23 spans. Assume that the optical nodes are each provided with a dispersion compensation module and that there are 21 kinds of candidate values which can be set to the respective dispersion compensation modules (i.e., the optical nodes are each provided with one of dispersion compensation modules having 21 different kinds of compensation amounts). In this case, the number of the candidate values of the compensation amount sums up to 2126. It is extremely difficult from a viewpoint of an amount of arithmetic operations to decide the presumably optimum one of the enormous number of the combinations.